1. Field of the Invention
The invention relates to a driving device, and more particularly to a light emitting diode (LED) driving device.
2. Description of the Related Art
FIG. 1 illustrates a conventional LED driving device 1 disclosed in U.S. Pat. No. 7,081,722. The conventional LED driving device 1 is used to drive a plurality of series connected LED units 10, each of which includes an LED. The conventional LED driving device 1 includes a rectifier 11 for rectifying an AC voltage from an AC power source (Vin) to a DC voltage (Vrec), a voltage generator 12 for generating a reference voltage (Vref), a plurality of switches (S1˜S4), a plurality of operational amplifiers (OP1˜OP4), and a plurality of resistors (R1˜R16).
In operation, initially, the DC voltage (Vrec) is not sufficient to turn on a first LED unit 10, and each of the first to fourth switches (S1˜S4) conduct in response to an output signal from a respective one of the operational amplifiers (OP1˜Op4). Then, when the DC voltage (Vrec) increases enough to turn on the first LED unit 10, the first to fourth switches (S1˜S4) still conduct such that a current flows through the first switch (S1) and the resistor (R1). When the DC voltage (Vrec) increases enough to turn on the first and second LED units 10, the operational amplifier (OP1) senses this condition by monitoring through the resistors (R3, R4) a potential (V2) at a common node between the second and third LED units 10, and turns off the first switch (S1). At the same time, the second to fourth switches (S2˜S4) still conduct such that a current flows through the resistor (R5) and the second switch (S2). Similarly, when the DC voltage (Vrec) increases enough to turn on the first to third LED units 10, the operational amplifier (OP2) senses this condition by monitoring a potential (V3) at a common node between the third and fourth LED units 10, and turns off the second switch (S2). When the DC voltage (Vrec) increases enough to turn on all of the LED units 10, the operational amplifier (OP3) senses this condition by monitoring a potential (V4) at one end of the fourth LED unit 10 distal from the third LED unit 10, and turns off the third switch (S3).
In such a configuration, the operational amplifiers (OP1˜OP4) serve as essential components to control operations of the first to fourth switches (S1˜S4). In addition, if the configuration of one LED unit 10 varies, for example, variation in the number or type of LEDs thereof, the reference voltage (Vref) generated by the voltage generator 12 must be adjusted accordingly.